Servovalves are used to magnify a relatively low power input signal (usually an electrical control signal in the order of a fraction of a Watt) to a high power hydraulic output (in the order of many thousands of Watts). Several types of hydraulic servovalves are known in the art- for example deflector jet, jet pipe and nozzle flapper. Each operates by using a pilot stage having a low power input to create a differential pressure at either end of a spool (the “main stage”). In many known applications the pilot stage is controlled by an electromagnetically driven armature, movement of which moves a small flow guide, flapper or jet pipe which in turn controls a differential pressure across a spool valve. The spool valve controls the flow of the high pressure working fluid. Servovalves typically comprise some kind of mechanical or electronic feedback system from the main stage to the pilot stage. For example, a feedback wire may extend from the spool to the armature to provide a torque opposing the electromagnetic force.
As described above, actuation of the pilot stage traditionally involved the use of an electromagnetic torque motor. Recently, piezoelectric actuation has started to emerge which offers a less complex and less expensive way of moving the flapper or flow guide of the pilot stage.
Known types of servovalves, in particular the deflector jet, jet pipe and nozzle flapper types have an intrinsic a quiescent flow whilst the valve is in the inactive or null state. In each type of valve, a proportion of the high power hydraulic flow is sent to a drain port without being used. This is undesirable as the energy in the fluid is not being used- therefore the valve loses some efficiency.